US-guided Sciatic Nerve Block: Evaluation of the Intraneural Ropivacaine Minimal Effective Volume (MEV90)

At least 45 patients undergoing hallux valgus repair will be enrolled. A baseline bilateral sciatic nerve electrophysiological study will be performed preoperatively.

Patients will be placed in prone position and a high-frequency (8-12 MHz) linear array transducer (Snerve, Sonosite, USA) will be used to identify the sciatic nerve in a transverse cross-sectional view at the apex of the popliteal fossa. As the best view of the sciatic nerve with its paraneural and neural sheath, just proximal to the point of bifurcation, will be obtained, the nerve dimension (medial-lateral and cranio-caudal diameters) will be determined and registered.

An 80 mm 22-gauge needle (Sonoplex, Pajunk, Germany) will be inserted using the US-guided in-plane technique under the epineurium. The positioning will be evaluated injecting 1 ml of sodium-chloride 0.9% solution and observing the nerve swelling, if correct the local anaesthetic (LA) solution will be injected.

The first patient will receive 15 ml of ropivacaine 1% (150 mg). Following patients will receive LA volume based on Up-and-down methodology.

Clinical follow-up will be performed at 12, 24, 48 hours, 1 and 5 weeks and 6 months after surgery.

At 5 weeks electrophysiological study will also be performed in all patients and repeated at 6 months in case of positive findings.

Inclusion criteria

• Age above 18 years old
• ASA I-III
• Signed informed consensus

Exclusion criteria

• ASA IV
• Unstable neurological disease
• DM type I-II
• Allergy to ropivacaine-mepivacaine
• Opioid chronic treatment
• Consensus refusal or not valid
• Rheumatoid arthritis
• Baseline electrophysiological study positive finding
• Postoperative intensive care required